There has been considerable interest in developing methods for preparing monodisperse polymer particles, i.e., having a narrow particle size distribution, particularly with particle sizes greater than about 2 microns. One polymer of particular interest in this application is polystyrene.
As disclosed in U.S. Pat. No. 4,247,434 which is incorporated herein by reference, microparticles in latex form find utility in a number of applications and yet other applications are awaiting the development of monodisperse polymer particles having a particle size above 2 microns. Such polymer particles are used as a reference standard for the calibration of various instruments, in medical research and in medical diagnostic tests.
The difficulty of preparing monodisperse polymer particles is evident from U.S. Pat. No. 4,247,434 in which the polymerization is conducted in microgravity, such as in outer-space.
This invention provides a process tool for assisting in achieving narrow particle size distribution and particle sizes greater than 2 microns by conducting multi-stage anionically catalyzed homopolymerizations under conditions selected to promote monodispersity.
Although multi-stage anionic polymerizations have been conducted to prepare block-copolymers, for example in U.S. Pat. No. 4,386,125, which is incorporated herein by reference, to our knowledge the anionic dispersion homopolymerization of monomers to prepare polymer particles having a particle size of greater than 2 microns and a narrow particle size distribution has not been previously proposed.